Tetrahydrobiopterin mediated mechanisms and therapeutic approaches in colitis
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BACKGROUND: Significant and paramount discoveries in the past two centuries have propelled studying the gastrointestinal tract (GI) and brain connection to the forefront of all research. As the second most ranked common inflammatory disease, inflammatory bowel disease (IBD) is a chronic disorder of the GI tract marked by periods of relapse and remission with symptoms of abdominal pain, weight loss, GI bleeding, and diarrhea. It affects as many as 3.1 million people in the US (1.3% of the total population) and 2.2 million people in Europe. In addition to contributing significantly to the morbidity, mortality, and disability demands on the health care system, IBD also considerably diminishes the quality of life due to its frequent reoccurrences, low remission rate and debilitating symptomology. Due to its chronic intestinal inflammatory state, pain (both inflammatory and referred visceral pain to the abdomen) is a defining symptom of the disease activity. However, due to its idiopathic nature, treatment of the inflammation and pain pathologies distinct to IBD has been unsatisfactory. Among the many treatments of IBD, sulfasalazine (SSZ) is an sepiapterin reductase (SPR) inhibitor of the tetrahydrobiopterin (BH4) pathway. However, SSZ has a more modest effect and is a less potent inhibitor when compared to other known SPR inhibitors, such as SPRi3, of which have proven to not be ideal in humans do to lack of solubility. Because SSZ is used as IBD treatment and because enterochromaffin (EC) cells, located mainly in the colon, produce over 90% of the body’s serotonin (5-HT), a neurotransmitter that requires BH4 for its synthesis, we questioned whether BH4 plays a critical role in the pathogenesis of IBD. Many studies have characterized EC cells as gate keepers to pain signaling, where they bridge the connection between intestinal contents and the somatosensory system located underneath that then connects back to the central nervous system. Our lab has further characterized BH4 as an essential key regulator for T-cell proliferation, a key mediator in IBD, by using a novel SPR inhibitor, QM385, to not only block its proliferation, but to also alleviate chronic pain. QM385 has the same inhibitory effects as SPRi3, but at a lower concentration with minimal side effects, longer half-life and more favorable central nervous system penetration kinetics in rodents. These groundbreaking findings allow us to hypothesize that BH4 plays a significant dual role in IBD inflammation and the resultant pain, by acting on: 1) Macrophage and T-cell activation and function and 2) 5-HT signaling from EC cells. Moreover, we hypothesize that the inhibition of BH4 with our newly created pro-drug, QM385, will reduce the sensory input from EC cells and the inflammatory response in IBD simultaneously. METHODS: The widely accepted DSS animal model was used to better understand the pathologies of pain and inflammation in IBD and to a deeper extent examine the effects of the QM385 treatment. Six-day 2% DSS treated mice and control were all were assessed for inflammation and pain using the following: disease activity index (DAI), abdominal hypersensitivity with von Frey filaments, colon microscopic lesions, levels of neopterin (BH4 pathway synthesis biomarker) and sepiapterin (SPR inhibition biomarker) and gene expression for GCH1 (BH4 pathway enzyme), TNF-α (inflammatory mediator), CD68 (macrophage marker), CD4 (T-cell marker), SPR (BH4 pathway enzyme), and TrpA1 (nociceptive ion channel expressed on EC cells). RESULTS/CONCLUSIONS: In this study, the DSS induced IBD model in mice was successfully implemented evidenced by the DAI data, morphological changes, and mechanical hypersensitivity. DSS treatment increased mechanical hypersensitivity, clinical and morphological inflammatory symptoms, BH4 activity (neopterin, GCH1, SPR), inflammatory mediators (CD68, CD4, and TNF-α), and nociceptive receptor transcription, TRPVA1 production. Sepiapterin was observed after QM385 treatment, suggesting BH4 inhibition of DSS-induced IBD. QM385’s role in alleviating the immune response in DSS-induced IBD was evident with the improvements in the macroscopic clinical symptoms of inflammation (DAI) and recovery in colon length. QM385 also demonstrated a significant improvement in referred visceral abdominal pain at 1 and 3 mg/kg. Moreover, QM385 appears to resolve the upregulation of 5-HT and GCH1 in the distal colon after DSS treatment; therefore, we show that QM385 suppresses the BH4 pathway in EC cells (i.e. suppress EC cell activation), by causing less 5-HT production and therefore, less activation of the gut sensory neurons. In summary, QM385 treatment alleviated the degree of colitis caused by DSS and the degree of inflammatory and referred visceral pain, suggesting that QM385 might be a novel agent for the treatment of chronic colitis as seen in IBD.